Towards detection and prevention of malicious activities . . .

The Internet has witnessed a tremendous growth the last years. Undoubtedly, its services and mostly the World Wide Web have become an integral part in the lives of hundreds of millions of people, who use it in daily basis. Unfortunately, as the Internet’s popularity increases, so does the interest of attackers who seek to exploit vulnerabilities in users ’ machines. What was primary meant to be mainly a competition among computer experts to test and improve their technical skills has turned into a multi-billion dollar business. Nowadays, attackers try to take under their control vulnerable computers that will allow them to perform their nefarious tasks, such as sending spam emails, launching distributed denial-of-service (DDoS) attacks, generate revenue from online advertisements by performing click-frauds, or stealing personal data like email accounts and banking credentials. In this dissertation, we address the security issues online users face every day from two points of view. First, we investigate how infected computers that constitu-te a botnet—network of compromised machines which are remotely controlled b